Unique sperm haplotypes are associated with phenotypically different sperm subpopulations in Astyanax fish.

BACKGROUND: The phenotypes of sperm are generally believed to be under the control of the diploid genotype of the male producing them rather than their own haploid genotypes, because developing spermatids share cytoplasm through intercellular bridges. This sharing is believed to homogenize their content of gene products. However, not all developing spermatids have identical gene products and estimates are that alleles at numerous gene loci are unequally expressed in sperm. This provides scope for the hypothesis that sperm phenotypes might be influenced by their unique haplotypes. Here we test a key prediction of this hypothesis. RESULTS: The haploid hypothesis predicts that phenotypically different sperm subpopulations should be genetically distinct. We tested this by genotyping different sperm subpopulations that were generated by exposing sperm to a chemical dye challenge (Hoechst 33342). Dye treatment caused the cells to swell and tend to clump together. The three subpopulations of sperm we distinguished in flow cytometry corresponded to single cells, and clumps of two or three. Cell clumping in the presence of the dye may reflect variation in cell adhesivity. We found that allelic contents differed among the three populations. Importantly, the subpopulations with clumped sperm cells were significantly enriched in allelic combinations that had previously been observed to have significantly lower transmission success. CONCLUSIONS: We show that at least one sperm phenotype is correlated with its haploid genotype. This supports a broader hypothesis that the haploid genotypes of sperm cells may influence their fitness, with potentially significant implications for the transmission of deleterious alleles or combinations of alleles to their offspring.

Sperm swimming behaviors are correlated with sperm haploid genetic variability in the Mexican tetra, Astyanax mexicanus.

The diploid genotypes of males are widely thought to determine sperm phenotypes, yet recent work shows that the haploid genetics of the individual sperm cell also contributes significantly. We tested seven sperm phenotypes, flagellar length and six behaviors, looking for correlations between genetic and phenotypic variability. While flagellar length appears to be controlled by the diploid genotype of the source, variation in three of the behavioral phenotypes, linearity, wobble, and progression are significantly correlated with the heterozygosity of the male producer. Because males that are more genetically variable produce a sperm set that is more diverse in its haploid genotypes, we suggest that the correlations may reflect significant haploid genetic control of sperm swimming behaviors.